Tool wear in cutting of Metal Matrix Composites.
Metal Matrix composites form one group of new engineering materials that have received considerable research since the first trial by Toyota in the early 1980s. Metal matrix composites offer various advantages in applications where high specific strength, stiffness and wear resistance [2-5] are required. MMC's are multiphase materials consisting of ceramic reinforcing fibres/whiskers/particles suspended in a metallic or inter metallic matrix.
The most common reinforcements are silicon carbide (SiC) and Alumina (Al2O3). Aluminium, magnesium and titanium alloys are commonly used as the matrix material.
In a conventional machining, a surface layer of the material is removed by a wedge shaped tool. There is plastic deformation of the work material ahead of the cutting edge due to entry of the tool edge into the work piece material and subsequent shearing. The machining of composites differs from metal machining significantly in some respects. The fibres are laid in parallel, anti-parallel and normal to the tool movement. There fore the physical properties of the composites together with fibre configuration and distribution of matrix determine the mach inability of composites.
In composite machining, three different Mechanisms operate. They are microploughing, micro-cutting, and micro-cracking. The harder tool wedge enters the soft matrix and removes the material by abrasion. When the cutting edge enters the work piece a combination of above mechanisms, i.e, plastic deformation, shearing and rupturing could take place. The mechanism of chip formation also differs with fibre angle in reference to cutting direction.
Understanding the tool wear mechanisms [6-8] establishes the basis for reducing or preventing tool wear, and hence enhancing the economics of metal cutting process as well as the produced surface integrity. In metal cutting tool wear can be broadly classified as physical and chemical wear. Physical wear includes: tool chipping, abrasion, attrition, and plastic deformation. Chemical wear, on the other hand includes oxidation wear, corrosion and dissolution.
At low cutting speeds and when adhesion tendency between the chip and tool material is low abrasion wear dominates. Attrition wear occurs when small particles from the tool material are removed by the sliding chip as the weak adhesive junctions between the tool are broken. The strength of adhesion bond or weld between the tool and the chip material increases as the cutting speed increases. This is attributed to higher cutting temperatures and strain levels in the chip produced at high cutting speeds. At high cutting speeds seizure occurs. As a result of seizure, shear localised chips are produced. Further more, the chip layer adjacent to the tool face becomes intensely deformed (secondary deformation zone) . Temperature at the tool/chip interface rises up as a result of the intense deformation. To obtain stress distribution under flank wear area which determine non uniform heat intensities between the flank and work piece interface [10-12].
If the tool material physical properties are such that heat dissipation from the tool tip is slow (i.e. low thermal conductivity and high specific heat) then the heat is accumulated in the tool tip area. These conditions could lead to thermal softening of the tool, that is to say the yield strength of the material is lowered to the extent to allow plastic deformation of tools is reported in several publications [3-4]. Yen et al. Made great progress in progressive flank wear and crater wear estimate with FEM code Deform-2D . Tool wear estimate with numerical methods is based on chip formation simulation and wear model are studied by various researchers [14-16].
In this study the turning test on Metal Matrix composites was performed with HSS and k-20 carbide tool material and wears patterns and wears land growth rates were analysed to evaluate the wear characteristics and classify the relationship between the physical properties and flank wear of cutting tool. The studies also extended to the machining aspects and width of cuts on Metal Matrix composites. When the cutting tool enters the work piece multiple cracking takes place ahead of the tool which results in shearing and rupturing. In this, the combination of the above three different mechanisms operate and this chip formation showed some similarities with the chip mechanism of metals.
Problems in machining of composites
The problems faced during machining of composites materials are numerous and differ from those of conventional metals. It has been observed that a HSS tool fails with in few seconds while machining composite materials. The low thermal conductivity of composites causes heat build up in the cutting zones which may also lead to thermal degradation of the matrix. These can be controlled by use of a coolant and also by machining at low feed rates. The use of a coolant depends on the properties of the work material.
Machining MMCs presents a significant challenge [5-8] to the industry since a number of reinforcement materials are significantly harder than the commonly used high speed steel (HSS) tools and carbide tools. In ceramics carbide tools, wear rate is low and decreases with increasing speed up to 300m/min, after which it increase. Tool wear rate is lowest in k-20 (Tungsten carbide tool) at all cutting speeds. A comparison of the different properties of cutting materials is shown in table 1. It is obvious from the table that Carbide tools are the best suited tools for machining of composites. Hence experiments were conducted with k-20 carbide tool.
Selection of cutting tools for machining
For machining Metal Matrix composites whose material and mechanical properties differ from regular metals in hardness, tensile strength, compressive strength and in wear resistance.
From the properties of tool material, a carbide k-20 tool bit has been selected for machining whose hardness, thermal conductivity, compressive strength, rupture strength, wear resistance and co-efficient of thermal expansion have found superior to the other conventional tools.
Experimental equipment and procedures
In this method, high speed HMT lathe (1450 mm x 800 mm x 5.5 kw) is used. The tested work material is Aluminum based metal matrix composite material with tubular shape having 25 mm inside diameter and 60 mm outside diameter having specific gravity 1.7 and volume fraction 70%.
The tested tool tips are square inserts (12.7mm x12.7 m x 4.7 mm SNP -432 type) with tool geometry of (-5degrees, -5 degrees, 5 degrees, 5 degrees, 15 degrees, 15 degrees, 0.8 mm) which are set in standard tool holder. The material of the tool tip is k-20 carbide tool with a composition of 94 % wc and 6% co.
The cutting conditions are as follows:
(1) A depth of cut of 1.0 mm, and feed rate of 0.1 mm/rev, remain constant through out all tests, several spindle speeds (i.e. cutting speeds) are chosen. The cutting fluid is used for washing off the chips.
(2) The flank wear lands of tested tools were measured with Olympu's STM tool maker's microscope. The profiles of the cutting edges at the nose were also measured by Toko Semitisu's Surfcom 2 B Surface meter. The work cutting edges were observed by Hitachi-Akashi's MSM-2 scanning electron microscope (SEM).
Experimental results and discussion
From tool wear patterns in machining MMC's, observed that there is a triangular nose wear and banded wear in carbide tools and also groove wear on ceramics and ceramites due to mechanical shaving action as shown in Fig.1 and Fig.2
Some experimental results on the growth of nose wear land with no. of cuts are shown under in Fig.3. The width of wear lands is almost proportional to the no. of cuts under the critical speed. In the vicinity of the critical speed, the wear land curve has small wear rate at the first stage and a large wear rate at the second stage. How ever, the sectional area curve of flank wear shows that its wear rates increase with no. of cuts. The wear rates on HSS tool and Carbide tipped tool are observed in stereo Microscope. It is observed that more wear rate has occurred in HSS tool rather than in Carbide tool. The land wear is shown in Fig. 4. It is observed by using a Stereomicroscope. In HSS tool at nose more wear and tear is observed and also minute cracks are observed at nose as shown in Fig. 5 & Fig. 6, in case of Carbide tipped tool the wear rate is very less and is shown in Fig.7 by observing with stereo Microscope.
Fig 8-10 show the effect of volume fraction and average particulate size of reinforcements on the progression of tool wear during cutting MMCs. As expected, the progression of tool wear is highly dependent on the volume fraction and average size of the reinforcements. It can be seen that as the volume fraction and average particle size increases, the rate of flank wear is increased. An increase in the volume fraction in- creases the number of hard particles coming into contact with the cutting tool. This results in a higher volume of tool material being lost due to the abrasion mechanism. When the particles are fine and well distributed in the matrix, their effect on the tool wear is slight and improves the machinability. It has been found that the coarser the particle, the more severe is the tool wear is, and hence the shorter the tool life.
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Cutting forces versus cutting speed
The cutting forces in aluminium based metal matrix composites is shown in Fig. 11. The cutting forces are reducing while increasing the cutting speed.
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Wear rate against spindle speed
The wear rate against the spindle speed is shown in fig.12. The primary wear is maximum at the speed of 250m/min and minimum at the cutting speed of 300 m/min. And secondary wear also maximum at 250 m/min and it is least at the speed of 300 m/min the cutting speed 250 m/min is critical speed at which grove wear and nose wear rate is maximum. This is attributed to the failure of cutting edge as a result of the softening of tool material with an increase of cutting temperature and it is concluded that the critical speed is proportional to the hardness at room temperature (the hardness is proportional to the compressive strength). Hence it is supposed that the compressive strength must play a significant role in the increase of the critical speed value, which shows the high speed cutting ability while machining of Metal Matrix Composites in cutting of a relatively high thermal conductivity materials, the heat transfer from the cutting zone to the tool is resisted with the use of a tool material with low thermal conductivity. The worn cutting edges observed from SEM will reveals that roundish banded wear in carbide tools, grooved wear, triangular wear in ceramites and ceramics tools. Under the critical speed, projector particles and gritty worn surfaces are observed. The worn cutting edges observed from SEM for HSS tool revealed that grooved wear exists heavily at the cutting edges.
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Further, the FEM code ANSYS LS-DYNA is used as the FEM calculation tool for chip formation, heat transfer and tool geometry updating, chip separation. ANSYS LS-DYNA is an Explicit general purpose code used for several formulations for numerical modelling: Lagrangian, Eulerian and Arbitrary Lagrangian Eulerian (ALE). Among them, Lagrangian and ALE formulations supply approaches to chip separation in the chip formation process. With ALE formulation, the mesh is not attached to the material and thus can move to update the free chip geometry and avoid distortion. Correct configuration of surface types and reasonable adaptive meshing control parameters, which depend on tool edge geometry, will ensure the smooth implementation of chip separation. With ALE, no predetermined separation line is required and coarser elements can still produce an acceptable chip thickness, cutting force, etc. Because no failure criterion is required, there is a broader selection of material models. The analysis of cutting process' steady state becomes the first step to the tool wear estimate. Here a complete modeling method from initial chip formation to the realization of steady state, which consists of three analysis steps, including initial chip formation, chip growth, and steady-state chip formation are included. The work is meshed with plane 162 (From Ansys Library) element type of 4025 elements. The cutting tool near cutting edge joins in the chip formation modeling, which consists of 615 elements. Moreover, in the first two steps, the cutting tool is defined as rigid body, whereas in the last analysis step the cutting tool has to be modeled as a deformable body in order to obtain the necessary cutting process variables for the latter tool wear estimate. The contact elements are used at the tool chip and work tool interface. A converged mesh is shown in Fig. 13. The work is fixed and the tool is moving in the negative x-direction. With the tool advancing into the work, elements along the concave surface extend and compose the chips outside surface. With a macro programme is developed in the Ansys software to extract the variables of the work and the tool about nodal coordinates, temperature, etc. are read into the model file of the chip growth analysis step from the output database of initial chip formation analysis step at a user-specified field output frame. In this step, the tool is fixed. The left and right boundary of the work are defined as Eulerian boundary regions, whose mesh is fixed in x-direction, but material flows in continuously from the left surface at cutting speed and flows out of the right surface. Fig. 14 shows the stress distribution at 1 ms. By adding the reaction force component in the same direction at all constrained nodes of the cutting tool and then taking the negative value, the cutting force components [F.sub.c] and [F.sub.t] are obtained. The mechanical cutting process variables, such as contact pressure, sliding velocity at 1 ms are read out for tool wear calculation. From orthogonal cutting experiment, in which the cutting depth is 1 mm and other cutting parameters have the same value as this simulated cutting condition, [F.sub.c] and [F.sub.t]. Fig. 15 shows the temperature distribution at 1 ms. The highest temperature is at rake/chip interface, and most part of the tool is still at room temperature. In this study, the heat produced in cutting process includes two parts, the heat created by the friction between the tool and the work, half of which is introduced into the tool, and the heat converted from inelastic energy, i.e., plastic deformation energy. Part of the latter heat is transferred to the tool through heat convection at tool/chip interface. The initial temperature value of the other nodes is set to room temperature. At the nodes on the tool/chip interface, heat flux is defined, and their value comes from the chip formation analysis step. In addition, the tool makes heat transfer with the environment through rake face and flank face. According to M.C. Shaw's equation of adhesive wear, Usui et al. deduced the characteristic equation of tool wear , given by
[??] = C vs [[sigma].sub.f] exp (-[lambda]/[[theta].sub.f]) (1)
where [??] is wear rate, i.e., the wear volume per unit area and unit time; [v.sub.s], the relative sliding velocity at tool/work interface; [[sigma].sub.f], the normal stress; [[theta].sub.f], the absolute temperature, C and [lambda] are constants determined for the combination of a tool and a work material. The latter study [17,18] shows that this equation is able to describe flank wear as well, which mainly results from abrasive wear. From previous analyses, cutting temperature and contact pressure have been obtained for every tool face node. At the tool face node, which loses contact with the work and the chip, relative sliding velocity is set to zero. After all the cutting process variables are obtained, tool wear rate is calculated at every tool face node using Eq. (1). Cutting time increment means the duration of cutting time between two successive tool wear measurements. If the tool wear is studied only with experimental methods, it is difficult to predict an approximate cutting time increment value for a specified tool wear increment value, whereas it is possible by using numerical methods before the tool wear curve is obtained since the wear rate is already known from the previous calculation. Wear value (The displacement vector of the tool face node due to wear) is calculated at every node on tool face by Eq. (2):
w = [??] * t * [w.sub.direction] (2)
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Numerical results & discussions
With this tool wear estimate approach, tool wear under the cutting condition described is calculated. The tool wear estimate process is accomplished with two calculation cycles. After the first calculation cycle, the new tool is updated to the worn tool and after the second calculation cycle, increased crater wear and flank wear can be found on the updated tool.
The solid line in Fig. 16 shows the wear progress curves of flank wear and crater wear obtained from experiment under the same cutting condition. The dot lines are predicted tool wear curves. It is found that the estimated flank wear and crater wear are smaller than experimental ones. In experiment, after 20 s of cutting, the flank wear has exceeded 0.15 mm and crater wear 0.06 mm, but after 46 s, the estimated flank wear just arrives at 0.1 mm and crater wear 0.03 mm.
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From these experiments on the tool wear in metal matrix composites the following conclusions have been derived.
(1) When cutting speed is increased, the nose wear will be in the shape of the triangle and its wear rate increases. The higher the thermal conductivity and compressive strength or smaller the thermal expansion coefficient, the higher the critical speed. The thermal conductivity is the most important among them as a factor to raise the critical speed.
(2) In cutting conditions of the critical speed, the groove wear on end of the cutting edge grows larger with decrease of cutting speed. The groove wears in the shape of round bands
(3) The wear rate is maximum at the cutting speed of 250 m/min and minimum at 300m/min.
(4) Finally, tool geometry is updated according to the calculated nodal displacements and one calculation cycle is finished, the program continues until tool reshape criteria is met. The user program continues until tool reshape criterion is reached. The number of calculation cycles carried on before user program stop is defined by dividing tool reshape criterion by the specified wear increment.
(5) Wear rate is simulated using Ansys LS-Dyna FEA Code.
Authors are thankful to UGC for their funding for conduct of experiments.
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Dr. P. Ravinder Reddy
Presently working as a Professor & Head, Dept. of Mech. Engg., Chaitanya Bharathi Institute of Technology, Hyderabad. Born on 12th August, 1965, Obtained B.Tech in Mechanical Engineering from Kakatiya University, and M.E in Engineering Design from PSG College of Technology, Coimbatore and did his Ph. D entitled Investigation of Machining parameters in orthogonal cutting and thermal stress variation of carbon epoxy matrix composites from Osmania University. Recipient of "Raja Rambapu Patil National award for Promising Engineering Teacher by ISTE for the year 2000 in recognition of his outstanding contribution in the area of Engineering and Technology. "Engineer of the year Award-2004" for his outstanding contribution in Academics and research by the Govt. of Andhra Pradesh and Institution of Engineers (India), AP State Centre on 15th September 2004. Received "UGC Fellowship" award by UGC (1999). Best Chief Coordinator with Excellence "A" Grade awarded by AICTE. The Biography is included in Marquis Who's Who in the World-2007 during Aug.2007. Published over 100 Technical papers in various journals and conferences. Principal Investigator for the AICTE Sponsored Project on "Fracture Based Design of Ceramic Matrix Composites and Fibre Geometric Modelling". Principal Investigator for the BHEL sponsored project on "Development of Test rig for Calibration of Overspeed Governor for Steam Turbines". Chief Coordinator for the UGC Sponsored Project on " Tool Wear and Orthogonal Cutting of Carbon Composites". Co-investigator for AICTE sponsored R&D project on "Integration of texture segments and optical character recognition for multimedia data bases. Chief Coordinator for the AICTE sponsored project on "Modernization of CAD/CAM and Establishment of FMS Lab. Principal Investigator for NSTL sponsored research project on Design of Composite propeller for higher cavitation Performance. Chief Coordinator, Industry Institute Partnership Cell (IIPC), Sponsored by AICTE. Life member of ISTE, ISME, ASQR, CMSI and Fellow of Institution of Engineers. Member of ASME and Charted Engineer.
A A Sriramakrishna
Presently working as a Associate Professor, Dept. of Mech. Engg., Chaitanya Bharathi Institute of Technology, Hyderabad. Born on 12th Sept., 1968. Obtained B.Tech in Mechanical Engineering from Kakatiya University, and M.Tech in Machine Tools from NIT Warangal, AP. Guiding both B.Tech and M.Tech projects and involved in curriculum development. Published over 10 papers in Journals and conferences. Actively involved in development of machine tool and production engineering laboratories. Pursuing research in machining of metal matrix composites.
(1) P. Ravinder Reddy and (2) A.A. Sri Rama Krishna
(1) Professor & Head and (2) Associate Professor (1,2) Department of Mechanical Engineering, Chaitanya Bharathi Institute of Technology Gandipet, Hyderabad--500 075, AP, India, (1) Email: email@example.com
Table 1: Physical Properties of cutting tool materials. Physical Property Carbide tipped Ceramics K20 P20 M10 [Al.sub.2] [0.sub.3] + TiC Hardness 93 91.5 92.5 92.93 Transverse rupture strength 1570 1570 1500 1300 Compressive strength, MPa 6080 4800 5000 3000 Youngs Modulus X [10.sup.5] MPa 6.9 5.4 5.8 3.6 Thermal Conductivity W/m-K 79 33.5 50 17.2 Coefficient of Thermal Expansion X[10.sup.-]6/C 5 6 5.5 8.3
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|Author:||Reddy, P. Ravinder; Krishna, A.A. Sri Rama|
|Publication:||International Journal of Applied Engineering Research|
|Date:||Nov 1, 2009|
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